Integration of bipolar transistor into CMOS technology results in a BiCMOS technology that offers higher flexibility in circuit design. BiCMOS technology offers increased functionality of the chips allowing for system on chip integration. Wireless communications represent large portion of consumer electronics market which is very sensitive to the cost of the product. Therefore, RF systems can gain benefits from integration of bipolar and CMOS technologies. In order to achieve the ability of system on chip integration, BiCMOS technology has to offer bipolar devices with different characteristics. In general, there is always tradeoff between speed and the breakdown voltage of the transistor. According to International Technology Roadmap for Semiconductors (ITRS), bipolar devices optimized for high speed, for high breakdown voltage and intermediate type of devices are needed for RF system on chip integration. In vertical bipolar transistors this is usually accomplished by different parameters of selectively implanted collector (SIC) for different types of transistors. This approach requires additional lithography masks and increases the complexity of the process and overall cost.
Vertical bipolar transistors are mainstream bipolar structures for BiCMOS integration. Process modules inserted in CMOS needed for bipolar transistor processing are as follows: deep trench isolations, buried collector layer, epitaxial collector region, collector plugs, base polysilicon layer, emitter base spacers, emitter polysilicon layer etc. Bipolar transistor can be made less complex by avoiding some of the mentioned process steps but at penalty of degraded electrical characteristics. On the other hand, additional process steps can be added such as Si/SiGe epi-base or raised extrinsic base in order to improve the performance. In general, processes are developed in the way that they are optimized as a cost performance or high performance processes.
Lateral bipolar transistors (LBTs) are also integrated into CMOS and are mainly realized on SOI wafers. In general, they require less additional steps to CMOS processing since the isolation is realized with buried oxide. Reduced process complexity and small parasitic capacitances make them attractive for low-power SOI BiCMOS applications. The drawback of the technology is use of SOI substrates which are more expensive as well as the processing of the intrinsic transistor which results in unoptimized doping profiles and characteristics that are inferior compared to vertical transistors.
The Horizontal Current Bipolar Transistor (HCBT) technology is demonstrated in U.S. Pat. No. 7,038,249 [1]. HCBT has a lateral arrangement of the intrinsic transistor, in the same way as in LBTs. However, the intrinsic transistor is processed on the sidewall of the silicon pillar or hill, obtained by etching, making it possible to have the optimized doping profiles in the intrinsic transistor. HCBT electrical characteristics are comparable to the characteristics of vertical bipolar transistors, with a considerable improvement over the LBTs, as has been reported in the literature [2]. Additionally, due to lateral arrangement of the intrinsic transistor, parasitic capacitances are smaller compared to vertical transistors. Furthermore, processing does not require deep trenches, epitaxial collector region, buried collector layer, collector plugs and base polysilicon layer. If HCBT structure is integrated into the BiCMOS technology, the integration requires a lower number of process steps to be added to CMOS technology. Moreover, the HCBT is normally processed by using bulk silicon substrate, but the HCBT concept can also be applied to SOI substrates. The simplified technology and the use of bulk silicon substrate make the HCBT technology a low-cost and simultaneously a high-performance solution for BiCMOS integration.
Integration of HCBT with standard 0.18 μm CMOS technology is published in [3]. HCBT structures for BiCMOS integration include high-speed (HS) [4] and high-voltage (HV) devices [5], which increases the flexibility of technology.